Concerns over the environmental consequences of burning fossil fuels have led to an increasing use of renewable energy generated from sources such as solar and wind. However, the intermittent nature of such renewable energy sources has made it difficult to fully integrate these energy sources into electrical power grids and distribution networks. A solution to this problem has been to employ large-scale electrical energy storage (EES) systems, which are widely considered to be an effective approach to improve the reliability, power quality, and economy of renewable energy derived from solar or wind sources. Among the most promising large-scale EES technologies are redox-flow batteries. Redox-flow batteries are special electrochemical systems that can repeatedly store and convert megawatt-hours (MWhs) of electrical energy to chemical energy and chemical energy back to electrical energy when needed.
A common electrochemical cell configuration in redox-flow batteries includes a positive electrode and a negative electrode in separated tanks and separated by an ion-exchange membrane, and two circulating electrolyte solutions, positive and negative electrolyte flow streams, generally referred to as the “catholyte” and “anolyte”, respectively. The energy conversion between electrical energy and chemical potential occurs instantly at the electrodes once the electrolyte solutions begin to flow through the cell. Redox-flow batteries can be recharged by inversing the flow of the redox fluids and applying current to the electrochemical reactor. Common redox-flow batteries are those based on redox systems comprising Fe3+/Fe2+ salts as anolyte and Cr2+/Cr3+ as catholyte. However, the ion-selective membranes separating the electrodes are not totally impermeable to chromium cations, and after some operation time, the chromium species diffuse in the iron compartment and vice-versa, decreasing the life-time of the redox-flow battery.
Even so, redox-flow batteries are the preferred large-scale EES technology since their capacity (energy) and their current (power) can be easily dissociated, and therefore easily scaled up. This is, energy can be increased by increasing the number or size of the tanks whereas the power is controlled by controlling the number and size of the current collectors rather than by changing the size of the electrolyte reservoirs. However both approaches imply using heavy and enormous tanks among other issues such as big pumps. Therefore, extensive research is being developed to optimize energy and power of the redox-flow batteries by using the right chemistry that maximizes the solubility of the redox couples in the electrolyte solution. The improvement of the solubility is crucial because of the enhancement of the mass transport of the reduced and oxidized ionic species leads to charge and discharge in the flow battery at higher current densities (power), and because higher concentration of redox ions leads to higher energy density of the cell. Vanadium-based redox chemistry, widely used in redox-flow batteries, exhibits solubilities of around 2 mol/l (1.26 V; 25-40 Wh/Kg). Although 8 mol/l of zinc iodide can be dissolved in zinc-polyiodide electrolyte-based systems, these are not totally redox-flow type as the deposition of solid zinc imposes a large electrochemical reactor [B. Li et al., Nature Communications 6, 6303 (2015)]. Similar limitations apply to zinc bromine systems (1.2 V; 80 Wh/Kg) [M. Skyllas-Kazacos et al., J. Electrochem. Soc. 158 (8) R55-R79 (2011)].
Li-ion based-chemistry has been also proved to increase energy density in redox-flow batteries, for example, using high-density slurries containing high- and low-voltage high capacity lithium ion intercalation compounds as catholyte and anolyte, respectively in an organic solvent [M. Duduta et al., Adv. Energy Mater. 1, 511-516 (2011)]. High viscosities and safety issues regarding the use of flammable organic solvents in the liquid electrolyte are salient disadvantages of the above systems. Alternatively using aqueous-based electrolytes containing alkali redox active ions in the catholyte side separated by an ion-conducting solid (ceramic) electrolyte from metallic alkali metal (i.e. Li or Na) have been proposed [Y. Lu et al., J. Mater. Chem. 21, 10113 (2011)]. However, the low conductivity of lithium ion through the ceramic electrolyte needs to be improved. Hence, aqueous-based redox-flow batteries, such as Vanadium (VRB) are still the most widespread chemistry in redox-flow batteries. However, some of the electroactive materials used in these aqueous redox-flow batteries are expensive, difficult to recycle, scarce and toxic (i.e. noble metals like Vanadium and the presence of highly aggressive chemicals like concentrated sulfuric acid).
Carbonylic compounds having acceptor (i.e. keto-carbonyl) or donor (i.e. enol/alcohol, amine-N—H, thiol) groups connected by a conjugated carbon-carbon skeleton that enables the delocalization of the n-electrons during the redox reactions have been also proposed as an alternative. These organic redox pairs undergo reversible and multiple proton-coupled electron transfer reactions having rate constants at least one order of magnitude higher than that of vanadium ions. However, two different organic molecules need to be used in order to have the acceptor and the donor functionalities at the same time [B. Yang et al., J. Electrochem. Soc. 161, (9) A1371-A1380, (2014)]. In addition, these compounds exhibit limited solubility in water [P. Fanjul-Bolado et al., Electrochim. Acta 53, 3635-3642, 2008]. This low solubility in water can be overcome by incorporating a sulphonic or hydroxyl functional group to the organic frame by simple chemical reactions. The presence of chemical substituents also allows tuning the standard reduction potential of the organic molecule and possibly enlarging the energy density of the flow cell. Recently, Aziz and co-workers [B. Huskinson et al., Nature, 505, 195-198 (2014)] have shown high-energy storage efficiency in a metal-free aqueous based flow battery by using low-cost quinone-disulphonic acid derivatives, more specifically an oxidized 9,10-anthraquinone-2,6-disulphonic acid as anolyte in a hybrid organic-inorganic cell. However, the use of bromine as oxidizer in this system is a hazard issue due to toxicity and corrosion. Anthraquinone-2,6-disulphonic acid and 1,2-benzoquinone-3,5-disulphonic acid as anolyte and catholyte, respectively, has been also published recently in a full organic cell [B. Yang et al., J. Electrochem. Soc. 161, 9, A1371-A1380, 2014]. In both documents, a two proton-coupled-electron transfer reaction was evidenced for a maximum solubility up to 1 M for the quinone molecules in acid media. In addition, and as for inorganic couples, there is not a full rejection rate of the membrane for both organic moieties, so the anolyte and catholyte will ultimately diffuse into each other, diminishing accordingly the energy density, thus increasing the size of the tanks needed.
The use of anthraquinone derivatives is also disclosed in WO2015/032480, particularly the compound anthraquinone-2-sulfonate, as a component of liquid electrolytes for electrochemical gas sensors for detection of NH3 or NH3-containing gas mixtures. Said anthraquinone derivative, as well as indigo polysulfonates, have also been used as redox mediators in aqueous redox electrolytes to ensure a good equilibrium between the redox centers and the working electrode in microbial fuel cells (Kengo Inoue et al., Appl. Env. Microbiol., 2010, 76(12), 3999-4007).
In view of above, there is a need for the development of low-cost and non-toxic redox electrolyte compounds with high water-solubility, fast kinetics and involving reversible and multiple proton-coupled electron-transfer redox reactions that could be used as integrality the anolyte or/and the catholyte, and thus reducing, the membrane leakage, in order to meet the market necessities.